In recent years, with miniaturization of a design rule, SOI wafers, each having an ultrathin film (i.e., an ultrathin SOI layer) which needs to be particularly high uniform, are used for SOI devices, such as FD-SOI (Fully Depleted SOI) devices, FinFET devices, and Si nanowire transistors. In these devices, the uniformity of SOI film thicknesses and buried oxide film (BOX film) thicknesses is important to determine the characteristics of transistors.
An conventional method for measuring film thickness distribution that calculates the film thickness distribution of a thin film of such a wafer with a thin film, the wafer with a substrate having a thin film on the surface thereof, generally performs film thickness measurement in each point by spectroscopic ellipsometry or reflection spectroscopy, but a film thickness distribution measurement unit that can perform high-throughput, high-precision film thickness distribution measurement on the whole surface of the wafer has not been commercially available.
In point measurement by spectroscopic ellipsometry or reflection spectroscopy, the spectrum in a certain wavelength range (in general, a visible light range) is obtained for each measurement point and fitting is performed on a model film structure with respect to the spectrum, whereby a film thickness in each measurement point is obtained. Therefore, an attempt to perform high-throughput and high-precision measurement on the whole surface of the wafer extremely increases the number of measurement points, which makes it practically impossible to perform measurement due to the amount of calculation and time constraint.
Moreover, a wavelength region with a wide wavelength range is required to perform spectral measurement, which makes it virtually impossible to increase the spatial resolution and perform multipoint film thickness measurement.
As described above, the challenge is to perform measurement of the film thickness distribution of a wafer with a thin film, such as an SOI wafer, at high density, with a high degree of precision, and in a short time.
Incidentally, in Patent Document 1, a method for measuring two layers: an SOI layer and a BOX layer at the same time by using a light of two wavelengths in an ultraviolet wavelength region by reflection spectroscopy is described.
In Patent Document 2, a technique of irradiating SOI with a white light, dispersing the reflected light according to wavelength, and calculating an SOI layer film thickness from wavelength-specific interference information is disclosed.
In Patent Document 3, for the purpose of measuring the film thickness of an object to be measured with a plurality of layers formed on a substrate with a higher degree of precision, a method for determining the film thickness of an object to be measured by fitting by using a light source having a wavelength component mainly in an infrared band and repeatedly calculating theoretical reflectance (spectrum) at each wavelength is described.